a practical guide to mems inertial sensors - stanford center for

A Practical Guide to MEMS Inertial SensorsStanford PNT SymposiumAlissa M. Fitzgerald, Ph.D. | 14 November 2013

10thanniversary

Stanford PNT Symposium © AMFitzgerald 2013

Overview

• About AMFitzgerald• How MEMS* are made• MEMS inertial sensor overview• Packaging• 6 DOF and more• Appendix: Specifications for a selection of sensors

*MEMS = Micro Electro Mechanical Systems


AMFitzgerald: Your Partner in MEMS Product Development


AMFitzgerald: MEMS solutions, from concept to production

Technology Strategy

Foundry Production

AMFitzgerald in-house Strategic partners

Design Simulation

Prototyping & Process Integration

Low Volume Production

Package & Test

© AMFitzgerald 2013

Our customer base is as diverse as MEMS itselfTypes of MEMS

developed in 2012:

Stanford PNT Symposium


Birth of MEMS

• Evolved from semiconductor processes

• 1970’s: using silicon processing to make mechanical devices, not transistors– Accelerometers– Pressure sensors– Inkjet nozzles

• 1982: Petersen’s “Silicon as a Mechanical Material”

Popular Science, June 1984



Start Finish

Silicon dioxide “Pulling” crystals Ingot

Semiconductor wafer

Solar cell wafer

Silicon – the purest material refined by humans



Silicon process technology

• Developed to make transistors and integrated circuits

Silicon wafer

Material depositionLithographic patterningMaterial etch backRepeat for multiple layers

Microprocessor

Images from: http://www.intel.com/education/chips/index.htm


Wafer dicing

• Similar to cutting tile



Die attach and wire-bonding


Die attach epoxy

25 micron (1 mil) gold wire


MEMS accelerometer schematic

• Displacement transduction method:– Capacitive– Piezoresistive– Thermal (MEMSIC)– Piezoelectric


In practice: diversity of architectures


ADI ADXL50 ST C5L24A

Accelerometers

3-Axis accelerometer


Source: ST, ChipworksiPhone 4 Teardown

MEMS gyroscopes schematic

• Rotation of component exerts perpendicular coriolisforce on resonating proof mass

• Displacement is measured capacitively


2-axis gyroscope


Source: ST, ChipworksiPhone 4 Teardown

3-axis gyroscope: introduced 2009


Package size: 4.4 x 7.5 x 1.1 mm

Source: ST, Chipworks

Bulk acoustic wave gyro – similar in concept to HRG


Precision single-axis with inductive drive/sense

Source: Silicon Sensing Systems


Bigger chips can span etch

zones

Edge of wafer etches faster

Performance challenges with MEMS inertial sensors

• Silicon etch uniformity affects all devices– Yield– Calibration– Performance binning


Source: Intellisense

Top heavy

Bottom heavy

Performance challenges with MEMS inertial sensors• Temperature sensitivity from CTE mismatch• Energy dissipation through structure and package

• Anchor design critical• Die attach materials• Lower Q, increased current consumption


DRV

net torque= 0

MEMS support

MEMS die

net torque

SNS

Used with permission from Tronics

By the way, these sensors need ASICs*!


Source: Chipworks photo of Kionix KXM52 Source: Chipworks photo of Bosch SMB380

Stacked Side-by-side

*Application-Specific Integrated Circuits


6 DOF “Combo” Sensors and more…

• Combo = accel, gyro, magnetometer, altimeter, etc.

• The Dream: Monolithic integration– Fabricate all sensors and control circuitry on same piece of

silicon– Small form factor, lower parasitics

• Reality today: Most are Multi-Chip Modules– Gyros need vacuum, accels need partial atmosphere– Best strategy to fight yield problems

ASICs yield at 98+%Accels yield at 60+%Gyros yield at 25+%

– Easier to swap out sensors and ASIC chips mid-product cycle

InvenSense Inc. Company Confidential

InvenSense vs. Competition

4x4x1 mm3

5 Die, including 1 vertical (Z-axis Magnet)>55 Wire bonds

MEMSGyro/Accel

GyroACIC

Accel/CompassASIC

X/Y AxisCompass

Z AxisCompass

3x3x1 mm³3 Die

<30 Wire bonds

Patented Single Cavity Multi‐Axis Motion Sensor Technologyenables Higher Integration and Smaller Size

InvenSense 9-Axis Competition 9-Axis

24Used with permission

24

Application grade and bias stability


Application grade Bias stability

Relative accuracy (*) Main application

CONSUMER AND AUTOMOTIVE GYRO

Consumer 10 °/s 3% Motion interface

Automotive 1 °/s 0.3% ESP

HIGH PERFORMANCE GYRO

Low-end Tactical also called Industrial

10°/h (Earth rate) 10 ppm Amunitions & rockets guidance

Tactical 1°/h 1 ppm Platform stabilization

Short-term Navigation 0.1°/h 100 ppb Missile navigation

Navigation 0.01°/h 10 ppb Aeronautics navigation

Strategic 0.001°/h 1 ppb Submarine navigation

Used with permission fromTronics

Range of current MEMS gyros

MEMS is only starting to compete on tactical grade performance


Summary

• MEMS fabrication technology enabled low-cost, small form-factor inertial sensors– Wide variety of device architectures and specifications– Broad adoption in consumer-grade applications in just 6

years due to low cost• Current sensor performance limited by current

architecture and manufacturing methods– Not likely to reach beyond Tactical Grade without technology

shift


Specifications

Silicon Sensing Systems – Single Axis High Performance


ST MEMS GyroscopesPart # Description Fullscale Main Features

L3GD20H 3-axis gyroscope in 3x3 LGA package.One single gyro for many applications: • Navigation• Location based services• Geo-fancing, • Gaming, • Pointing devices, etc.

• ±245dps• ±500dps• 2000dps

• Very Low noise (0.011 dps/√Hz)• Ultra Low Power Consumption• Embedded Power down• Embedded Sleep Mode• 16-bit Resolution • I²C/SPI interfaces• Embedded FIFO• Embedded Temp. Sensor• Wide V. Range: 2.2V to 3.6V• Low voltage compatible IOs, 1.8

V

L2G3IS 2-axis OIS gyroscope in 3x3.5x1LGA PackageApplications: • optical image stabilization• Applications requiring very high

sensitivity and ultra low noise.

• ±65dps• ±130 dps

• Ultra Low noise (0.006 dps/√Hz)• Low Power Consumption• Power-down and Sleep Mode for

smart power Saving• SPI digital interface• Embedded temp. sensor• Integrated low and high-pass

filters with user-selectablebandwidth

• Wide V. range: 2.4 V to 3.6 V• Low voltage-compatible IOs

(1.8V)


ST MEMS Inertial ModulesPart # Description Fullscales Main Features

LSM6DS0 iNEMO inertial module in 3x3:• 3-axis accelerometer• 3-axis gyroscope

Accel: ±2/±4/±8 g Gyro: ±245/±500/±2000 dps

• Embedded Power-down • Embedded Sleep modes• SPI/I2C serial interface• Embedded Temp. Sensor• Embedded FIFOs • V. Range: 1.71 V to 3.6 V• Independent IOs supply (1.8 V)• Embedded Selftest

LSM9DS0 iNEMO inertial module in 4x4 LGA Package:• 3-axis accelerometer• 3-axis gyroscope• 3-axis magnetometer

Accel: ±2/±4/±6/±8/±16 g Mag: ±2/±4/±8/±12Gyro: ±245/±500/±2000 dps

• SPI / I2C serial interfaces• V. Range 2.4 V to 3.6 V• 16-bit data output• Power-down / low-power mode• Programmable interrupt

generators• Embedded self-test• Embedded temperature sensor• Embedded FIFO• Position and motion detection• Click/double-click recognition• Intelligent power saving for

handheld devices


ST Compass ModulesPart # Description Fullscales Main Features

LSM303C/LSM303E

E-compass in 2x2 LGA Package• 3-axis accelerometer• 3-aixs magnetometerApplications:• Tilt-compensated

compasses• Motion-activated

functions

Accel: ±2/±4/±8 g Mag: ±16 Gauss

• Ultra-compact • High-performance• SPI / I2C serial interfaces• 16-bit data output• Analog supply voltage 1.9 V to

3.6 V• Power-down mode / low-power

mode• Programmable interrupt

generators• Embedded temp. sensor• Embedded FIFO


Example of a high performance MEMS Gyro

Z-axis Angular Rate Sensor

300o/s input range, 100 Hz BandWidth

24-bit output (SPI)

Bias Instability (Allan variance): 1o/h

Bias variation over temp (-40/+85oC):

+/- 0.05 o/s

Noise density: 10o/h/√Hz

Power consumption: 25mA under 5V

19.6 x 11.5 x 2.9 mm

2 grams

GYPRO2300 released atElectronica, Nov 2012

Used with permission

a practical guide to mems inertial sensors - stanford center for

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